Transcript for Hurricane Force - A Coastal Perspective, segment 04 of 12

Deciphering hurricane-related coastal change calls on a basic understanding of how hurricanes form and progress. Hurricanes are part of a family of storms called tropical cyclones. Those impacting the United States typically develop between June and late November just north of the equator in the tropical to subtropical waters of the Pacific Ocean, the Gulf of Mexico, the Caribbean Sea, and the Atlantic Ocean. Hurricanes generally form in zones of low pressure where moisture-laden warm air converges, feeding clusters of thunderstorms. In Earth's northern hemisphere, the growing cloud mass always rotates in a counterclockwise spiral, a process known as the Coreolis Effect. As wind speeds exceed seventy-four miles per hour, a hurricane is born. Hurricane intensities range from a Category One hurricane with winds between seventy-four and ninety-five miles per hour up to a Category Five hurricane with winds greater than one hundred fifty-five miles per hour.

The central low-pressure eye of a hurricane is from four to forty miles across. It is surrounded by a cylindrical band of highest winds called the eye wall. Air at the sea surface is sucked towards the eye and thrust upwards in the eye wall. As it rises, the moist air cools, causing water to condense as a fine mist or ice while releasing huge amounts of heat. An average hurricane releases heat equivalent to the total electrical energy consumed annually in the United States.

Tremendous volumes of rain are often a by-product of this energy transfer, commonly producing flash floods at landfall. Hurricanes are pushed by the prevailing winds until cold water or land cuts the storm's energy supply, causing it to weaken and die. The right semicircle of a hurricane is particularly destructive. Here, the forward motion of the storm adds speed to the counterclockwise-spinning winds. These high winds combine with the storm's low atmospheric pressure, producing elevated sea level that moves as a lenslike bulge beneath the storm. At landfall the bulge in sea level becomes a life-threatening flood called a storm surge which can extend inland many miles, reaching over twenty-five feet above mean sea level. Waves and currents working with the storm surge cause most of the coastal geologic change attributable to hurricanes.

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